Spark plug and method of making the same



Feb. 13, 1940. MARGUN 2;]90269 SPARK PLUG AND METHOD OF MAKING THE SAME Filed D86. 7, 1937 f %INVENT OR ATTO RNEYS Patented Feb. 13, 1940 UNITED STATES PATENT OFFICE SPARK PLUG AND METHOD OF MAKING THE SAME George Marglin, Port Washington, N. Y., as-

signor, by direct and mesne assignments, to

Aeroflex Laboratories, Incorporated,

Long

'11 Claims.

This invention relates to spark plugs and method of making the same, and has for an object to contrive an economical, highly efficient and practical spark plug which is capable of withstanding the relatively sudden and wide'temperature variations and pressures in particularly airplane engines to which such engines are subjected.

Another object is to provide a structure which will be well adapted to avoid overheating and physical breakdown even under exacting conditions imposed in heavy duty engines, such as those employed in airplanes and in racing automobiles.

Another object is to provide a spark plug in which the insulation between the sparking electrodes is compressible to a certain degree and the center electrode inserted with a driving fit, thereby forming a gas tight joint therebetween and preventing rupture of the parts when being assembled.

Another object is to provide an improved method of manufacture of spark plugs which will be relatively inexpensive to construct and capable of being easily assembled.

A still further object is to provide certain improvements in the form, construction and arrangement of the parts whereby the above named and other objects may effectively be attained.

A practical embodiment of my invention is represented in the accompanying drawing, in which Fig. 1 represents a vertical central section on an enlarged scale of my improved spark plug and its insulated conductor removably attached thereto, and r a Fig. 2 represents a modified form of central electrode assembly. v

It iswell known that the present day engines require a greater factor of safety and emciency in spark plugs brought about by the increase in the horse-power and compression. The spark plugs are therefor called upon to withstand terrific heat of the lean mixtures of gasoline and the rapid changes in temperatures especially in airplane engines in which the changes may range from 120 F. on the ground to below zero above the clouds; oil flushing when the cooling of the engine is necessary. Furthermore, constant vibration places a severe strain on a spark plug, while the high tension current flowing through the spindle is also a rigid test on the insulating qualities of the spark plug. It is with these known factors in mind that the present invention was conceived.

In the form shown in Fig. 1 the metal shell of the plug is denoted by l and is screw threaded exteriorly at 2 for insertion into the engine casting (not shown). The shell is provided with a longitudinal bore 3 having its upper end interiorly screw threaded at.4 for the reception of a nut 5, and having an interior annular flange 6 at its lower end. A spark electrode 1 is located in the flange to extend inwardly therefrom to form one terminal of the spark gap. A cylindrical central electrode 8 having a spark tip 9 dependsfrom the shoulder ill of the electrode to form the other terminal of the spark gap. This central electrode 8 is tapered gradually and diminishes in diameter from the shoulder I to the upper end II, which is countersunk at l2 and terminates in a cup l3. The central electrode 8 is snugly wrapped with a sheet of compressible, heat resisting insulating material such as mica to form a multiple-ply tube l4, which is cut off flush with the shoulder ID and arranged to extend above the upper end I I of the tapered electrode to form a shield for preventing the creeping of high tension currents which might occur when the plug is overheated. Since this sheet is wrapped around the tapered electrode, it will be seen that this tube I4 likewise assumes a tapered form. An insulator i5 formed of a number of washers of suitable insulating material such as mica, which have been previously prepared, inserted in the cylindrical bore 3 of the shell I and tightly held between the annular flange 6 and. the nut 5. This insulator I5 is provided with a tapered bore l6, while the nut 5 has a corresponding taper l'l into which the tapered insulating tube It is inserted for the reception of the tapered electrode 8 which is driven therein to form a gas tight joint therebetween. In order to prevent leakage between the insulator I5 adjacent the annular flange 5, I rabbet the lower end of the washers at l8 and wrap therein a sheet of insulating material, such as mica to form a multiple-ply tube l9. Before installing the insulator l5 in the cylindrical bore 3 of the shell I, it is prepared in the following manner:

A predetermined number of washers of insulating material such as mica, having a diameter larger than the interior diameter of the cylindrical bore 3 of the shell are assembled on a straight mandrel and compressed under immense pressure to the length desired, in any well known and approved manner. The edges of each washer are usually very rough and easily split or splintered and in order to obtain a tight fit in the cylindrical bore 3 it is-necessary to machine and polish the edges to the desired di- I ameter, and this result can only be accomplished when a number of washers are compressed into a roll.

This rough surfaced roll of compressed washers is then machined and polished to the desired diameter or to the interior diameter of the cylindrical bore 3 of the shell I. The rabbet i8 is then machined, polished and wrapped with the sheet of mica to form the multiple-ply tube l9, after which the insulator with the mandrel still in position is driven into the cylindrical bore 3 in the shell I and the nut 5 screwed thereinto to lock the insulator in place. The mandrel may now be removed from the insulator and the cen tral hole, in which the mandrel was located, reamed and polished to a size suitable to receive the tapered insulating tube l4 and its tapered electrode 8, which is driven therein to form a gas tight joint. Simultaneously with the reaming and polishing of the insulator I5 the annular flange 6 and nut 5 may be reamed and polished if desired. After the tapered electrode has been inserted the upper and the lower ends may be machined to the desired form and the spark terminals prepared in the usual manner. The width and depth of the firing cavity 20 may also be varied to suit difierent requirements.

The hightension current is transmitted to the spark plug by a sheathed rubber insulated lead-in wire 2| arranged to depend into the cup l3 of the tapered electrode 8. This wire is secured in posi-- tion in a metal cap 22 by means of a bushing 23 screw threaded in the upper end of the cap 22. An insulator 24 is disposed within the cap to receive the end of the wire 2| which extends through a depending tip 25 of the insulator so as to project into the upper end of the multiple-ply tube l4 above the nut 5 and tapered electrode 8. The cap 22 is arranged to snugly fit the upper end of the shell I and is yieldingly held in position by a split ring 26 seated in an annular channel 21 in the shell.

In the form shown in Fig. 2 the construction and method of preparing and assembling t .9 parts are similar to that described with respect to Fig. 1, with the exception that the insulator 28 is reamed and polished from the top through the nut 29 to form a tapered bore 30, so that the diameter of the bore will gradually diminish toward the firing chamber 3|.

Likewise, the cylindrical central electrode 32 will be tapered to correspond to the taper of the bore 30 when wrapped with the sheet of mica to form the multiple-ply tube 33. In this form the tapered electrode 32 will be driven into position in the mica tube 33 to form a gas tight joint from the upper end of the nut 29 and insulator 28.

From the foregoing it will be seen that the spark plug of this type of construction is void of any air pockets in which moisture might collect and create static sparks, and the joints of the structure absolutely gas tight and free of any unintentional leakage therethrough either electrical or mechanical.

The parts shown and described but not claimed herein form the subject matter of my divisional application filed June 21, 1939, Serial No. 280,277.

I It will be understood that various changes may be resorted to in the form, construction, and arrangement of the several parts without departing from the spirit and scope of my invention, and hence I do not intend to be limited to the particular embodiments hereinshown and described except as they may be included in the claims.

What I claim is:

1. A spark plug comprising, a metal shell having a longitudinal bore interiorly screw threaded at its upper end, a relatively longitudinally yieldable cylindrical insulator fitted snugly within said bore and provided with a firing cavity, means screw threaded in the bore below the top of the shell and engaging the insulator for securing it in the bore and a longitudinally tapered electrode driven in said insulator and forming a gas tight joint therebetween.

2. A spark plug comprising, a metal shell having a longitudinal bore interiorly screw threaded at its upper end, a relatively longitudinally yieldable cylindrical insulator fitted snugly within said bore and provided with a firing cavity, the interior wall of said insulator being tapered, means screw threaded in the bore below the top of the shell and engaging the insulator for securing it in the bore and a longitudinally tapered electrode driven in said tapered interior wall of the insulator and forming a gas tight joint therebetween.

3. A spark plug comprising, a metal shell having a longitudinal bore interiorly screw threaded at its upper end, a relatively longitudinally yieldable cylindrical insulator fitted snugly within said bore and provided with a firing cavity, means screw threaded in the bore below the top of the shell and engaging the insulator for securing it in the bore, a tubular roll of mica inserted in the bore of the insulator arranged to extend from the top of said cavity throughout the length of the bore in the insulator, and a longitudinally tapered electrode driven in said tube of mica and forming a gas tight joint therebetween and between the tube and insulator.

4. A spark plug comprising, a metal shell having a longitudinal bore interiorly screw threaded at its upper end, a relatively longitudinally yielciable cylindrical insulator fitted snugly within said bore and provided with a firing cavity, means screw threaded in the bore below the top of the shell and engaging the insulator for securing it in the bore the interior wall of said insulator being tapered, a tubular roll of mica inserted in said tapered interior wall of the insulator and arranged to extend from the top of said cavity throughout the length of the insulator, and a longitudinally tapered electrode driven in said tube of mica and forming a gas tight joint therebetween and between the tube and insulator.

5. A spark plug comprising, a metal shell having a longitudinal bore interiorly screw threaded at its upper end,,a relatively longitudinally yieldable cylindrical insulator composed of a plurality of mica washers compressed longitudinally and extending substantially throughout the length of said bore, a firing cavity in said insulator, means screw threaded in the bore below the top of the shell and engaging the insulator for securing it in the bore and a longitudinally tapered electrode driven in said insulator and forming a gas tight joint therebetween.

6. A sparkplug comprising, a metal shell having a longitudinal bore interiorly screw threaded at its upper end, a relatively longitudinally yieldable cylindrical insulator composed of a plurality of mica washers compressed longitudinally" and extending substantially throughout'the length of said bore,.a,firing cavity in said insulator, the

interior wall of the insulator being tapered,

I means screw threaded in the bore below the top electrode driven in said insulator and forming a gas tight joint therebetween.

7. A spark plug comprising, a metal shell having a longitudinal bore interiorly screw threaded at its upper end, a relatively longitudinally yieldable cylindrical insulator composed of a plurality of mica washers compressed longitudinally and extending substantially throughout the length of said bore, a firing cavity in said insulator, means screw threaded in thebore below the top of the shelland engaging the insulator for securing it in the bore a tubular roll of mica inserted in the insulator and arrangedto extend from the top of the said cavitythroughout the length of the insulator, and a longitudinally'tapered electrode driven in said tube of mica andforming a gas tight joint therebetween and between the exterior longitudinal surface of the tube and interior bore of the insulator.- v

,8. A'spark plug-comprising, a metal shell having a longitudinal bore interiorly screw'threaded at its upper end, a relatively longitudinallyyieldablecylindrical insulator composed of a plurality of mica-washers compressed longitudinally and extending substantially throughout the length of said bore, a firing cavity in said insulator, means screw threaded in the bore below the top of the shell and engaging the insulator for securing itinthebore, the interior wall 01' the insulator being tapered, a tubular roll of mica inserted in the insulator and arranged to extend from the top of said cavity throughout the length of, the insulator, and a longitudinally tapered electrode driven in said tube 01' mica and forming a gas tight joint therebetween and between the exterior longitudinal surface of the tube and interior bore of the insulator.

9. A spark plug comprising, a metal shell hay a ing'a lonigtudinal bore and an annular flange at the lowerv end, a cylindrical insulator composed of a plurality of mica washers compressed longitudinally and extending substantially throughout the length of said bore of the shell and having a tapered bore, the exterior surface at the lower end of the insulator being rabbeted, a multiple ply tube of mica disposed in said rabbet to form a gas tight joint between the shell and the insulator, a 'flring cavity in the insulator, a tubular roll of mica inserted in the tapered bore of the 1 l insulator arranged to extend from the top of said cavity throughout the length of the insulator,

lower end, a nut screw threaded in said threaded bore, a cylindrical insulator composed of a plurality of mica washerscompressed longitudinally and extending substantially throughout the length of said bore of the shell and having a tapered bore, said insulator being interposed in the bore of the shell between the flange and nut, the exterior surface at the lower end of the insulator being rabbeted, a multiple ply tube of mica disposed in said rabbet to form a gas tight joint between-the shelland insulator, a flring cavity in the insulator, a tubular roll of mica inserted in the tapered bore of the insulator arranged to extend from the top of said cavity throughout the length of the insulator, and a longitudinally tapered electrode driven in said tubular roll of mica to compress the roll against the tapered bore of the insulator and form a gas tight. joint between the electrode and roll of mica.

11. A spark plug comprising, a metal shell having a longitudinal bore interiorly screw threaded at its upper end, a relatively longitudinally yieldable cylindrical insulator composed of a plurality oi mica washers compressed longitudinally and extending substantially through the, length of said bore, a multiple ply tube of mica disposed in the bore at the lowerend of the insulator to form a gas tight joint therebetween, a flring cavity in the insulator, means screw threaded in the bore below the top of the shell'and engaging the insulator for securing it in the bore, the in- V .terior wall or the insulator being tapered, a tubular roll of mica inserted in the-tapered interior wall of the insulator and arranged to extend from the top of said cavity throughout the length of the insulator, and a longitudinally tapered f electrode driven in said tube of mice. and forming a gas tight Joint therebetween and between the exterior longitudinal surface of the tube and interior bore of the insulator.

GEORGE MARGLIN. 

